(MoneyWatch) American homes are getting a little congested.
An estimated 51.5 million people live in multi-generational housing, which typically means three generations under one roof. That number is expected to increase as baby boomers get older. At least 10,000 Americans will turn 65 each day for the next 19 years, according to the Pew Research Center.
Whether to take care of elderly parents or just to make ends meet for young couples and parents, more families are moving in together. The Williams family is a case in point. When Art Williams' mother was injured in a work accident, he and his wife decided to have her move in with them. While he was happy to take care of his mom, and she was happy to take care of his two young children while he and his wife worked, the arrangement was a crowded one.
"It was our first time living with someone else, and it was definitely different," says Williams, 33. "When she first moved in, my son was four or five, and he kept asking when Grandma was going to go home."
But last September, Williams found a solution that made his entire family -- even his young son -- happy. He bought a NextGen home from Lennar, a national builder that offers multi-generational housing in the Los Angeles area.
"The concept is to create a home-within-a-home," says Jeff Roos, a Lennar regional president. "You have a full home with a separate, private apartment attached to it."
NextGen homes look like any other, but each includes a separate "near" apartment, featuring a bedroom, bathroom, living area and kitchenette. It is "near" rather than "full" because it lacks a stove, which would technically turn the separate space into a new residence in most zoning codes. The apartment even has its own entrance.
The idea was born out of a trend that took hold during the recession, where families doubled up to save on housing costs. Kids who had run out of money moved back into childhood homes, heads of families who had lost their homes to foreclosure moved in with siblings, or mom and dad settled in with their children's families.
"When that occurred, people lived in a guest bedroom, sometimes a dual master suite, but they didn't have an independent living space where they can hang out, cook, watch TV, play games, do their laundry," Roos says. "They're not just living together; they're living on top of each other."
Roos' concept isn't entirely new. The American Institute of Architects has been tracking the popularity of separate living space for years.
"There was term called 'accessory apartments' that dates back quite a while," says Kermit Baker, chief economist for the AIA. "I think it was a popular concept 20 to 25 years ago that expanded out to au pair or in-law suites."
The trend wound down over the years, but it picked up speed again when the economy took a nosedive. In the AIA's most recent annual survey of architects, 26.7 percent said the design concept was on the rise.
"It's something that's on the radar screen for designers," Baker says. "But I wouldn't say it's blasting off in popularity. Instead, there's a clear niche for this."
Lennar has tapped that niche and has found success across the country with its NextGen designs. The company first rolled out the homes in Phoenix in 2011, and it has since extended the concept to 120 communities in 10 states. Lennar is betting that demand will continue to increase.
"This is a product that is not just a recession buster," Roos says. "It has the ability to change some lifestyles for a long period of time."
Their NextGen home certainly changed the lifestyle of the Williams family. Williams' mother spends her days with the family in the main house, but the three generations separate at night and get much-needed alone time. They can now choose how to spend their weekends: as a group or on their own.
"For us, it wasn't that we couldn't have lived all together, but this is so much better," Williams says.